Polytetramethylene ether glycol is a commodity in the chemical industry, widely used to form segmented copolymers with polyfunctional urethanes and polyesters. It is commonly prepared by reacting tetrahydrofuran with a strong acid catalyst such as fluorosulfonic acid and then quenching the product with water.
While this process has been proved to be quite satisfactory, it is not as efficient as desired because the acid catalyst cannot be recovered and reused. Moreover, disposal of the spent acid catalyst is a problem because of its toxicity and corrosiveness.
More recently, as described in U.S. Pat. No. 4,120,903, it was found that polytetramethylene ether glycol can be prepared from tetrahydrofuran using a catalyst which is a polymer containing pendant fluorosulfonic acid groups and a chain terminator which is water or 1,4-butane diol. The nature of the catalyst permits its reuse, thereby eliminating the disposal problems, and the catalyst's lack of solubility in the reaction mass makes it easy to separate the catalyst from the product at the end of the polymerization reaction. This very low solubility also minimizes loss of catalyst as the reaction proceeds. However, this process produces a product, polytetramethylene ether glycol, having a molecular weight of 10,000 or more, while the commercial products generally have molecular weights of less than 4,000 with the majority of commercial products having a number average molecular weight of either 1,000 or 2,000.
More recently U.S. Pat. No. 4,163,115 disclosed that the molecular weight of the polytetramethylene ether glycol product, when using a catalyst which is a fluorinated resin containing sulfonic acid groups, can be controlled by adding an acylium ion precursor to the reaction medium. The acylium ion precursors were aryl halides and anhydrides of carboxylic acids whose carboxylic acid moieties contain 1 to 36 carbon atoms and especially those of 1-4 carbon atoms. Acetic anhydride, propionic anhydride and formic-acetic anhydride were specifically illustrated. Examples 10-13 of the patent disclose the use of mixtures of acetic anhydride and acetic acid as molecular weight control agents. The reaction product is an ester capped polytetramethylene ether which is reacted with an alkanol such as methanol to provide the final product polytetramethylene ether glycol which, in the preferred case, yields methyl acetate as a by-product.